The eukaryotic cell cycle is controlled by protein complexes composed of cyclins and cyclin-dependent kinases (cdks). The regulatory function of cdks is achieved by phosphorylation of key substrates, such as the members of the retinoblastoma gene family. Activity of cdks is regulated by post-translational modification and by the association or dissociation with inhibitory subunits designated cyclin-dependent-kinase inhibitors (CKIs). Two families of these inhibitors have been identified in mammalian cells. Members of each of the two families share a high percentage of sequence homology, in addition to their specificity of interaction with cdks. The first family, which includes p21 (also known as Cip1, Pic1, Sdi1, mda6 and Waf1), p27 (also known as Ick, Kip1 and Pic2) and p57 (also known as Kip2) preferentially inhibit cdk2. The second family, which includes p16 (also known as Ink4A, Mts1, Cdkn2 and Cdkn4i), p15 (also known as Ink4B and Mts2), p18 (also known as Ink4C and Ink6A) and p19/p20 (also known as Ink4D and Ink6B) preferentially bind to and inhibit cdk4 and cdk6. Results from several studies indicate CKIs to be the products of potential tumor-suppressor genes (MacLachlan et al. (1995) Crit. Rev. Eukaryotic Gene Expression 5:127-156).
p27 was first identified in complexes with cdk2/cyclin E in Transforming Growth Factor-.beta. (TGF-.beta.) arrested cells (Koff et al. (1991) Cell 66:1217-1228). p27 protein associates with cyclin E/cdk2 and Cyclin A/cdk2 complexes and inhibits their activities. It is a negative cell cycle regulator implicated in G1 phase arrest by TGF-.beta. cell contact, inhibition agents that increase the level of cyclic AMP, staurosporin, lovastatin, tamoxifen and rapamycin. Overexpression of p27 protein in mammalian cells induces a G1 block of the cell cycle (Polyak et al. (1994) Cell 79: 59-66; Toyoshima, T. and Hunter, T. (1994) Cell 78: 67-74). In addition, high levels of p27 have been found in quiescent cells thus suggesting a role for p27 in maintaining cells in G0 (Nourse et al. (1994) Nature 372:570-573). Levels of p27 decrease as cells reenter the cell cycle, mostly due to ubiquitin-proteosome dependent degradation (Pagano et al. (1995) Science 269:682-685). No structural alteration of p27 gene has been reported in human neoplasms to date (Bullrich et al. (1995) Cancer Research 55: 1199-1205; Cordon-Cardo, C. (1995) Am. J. Pathol. 147:1-16; Kawamat et al. (1995) Cancer Research 55:2266-2269; Pietenpol et al. (1995) Cancer Research 55:1206-1210; Ponce-Castaneda et al. (1995) Cancer Research 55:1211-1214; Stegmaier et al. (1996) Cancer Research 56:1413-1417). Thus, despite its potential role as a tumor suppressor, p27 gene does not appear to be mutated in human tumors.
However, recent evidence suggests an involvement of this CKI in neoplastic transformation. For example, p27 deficient mice have been shown to develop tumors of the pituitary gland with 100% of penetrance (Fero et al. (1996) Cell 85:733-774; Kiyokawa et al. (1996) Cell 85:721-732). In addition, p27 has been shown to be a target of the Adenovirus E1A (Mal et al. (1996) Nature 380:262-265). Further, HPV E7 oncoproteins have been shown to dissociate p27 from the cyclin/cdk complexes (Zerfass-Thome et al. (1996) Oncogene 2323-2330). p27 has also been implicated as a regulator of drug resistance in solid tumors and has been suggested as a target for development of antagonists which may be useful as chemosenstizers in conjunction with conventional anticancer therapy. St. Croix et al. (1996) Nature Medicine 2(11):1204-1210.
It has now been found that levels of p27 expression in tumor cells correlate with the degree of malignancy of the tumor and can be used in prognosticating overall survival time in cancer patients.